Giải câu 8 bài 2: Hình thang sgk Toán Hình 8 tập 1 Trang 71.
Ta có : AB // CD
=> $\widehat{A}+\widehat{D}=180^{\circ}$ ( góc trong cùng phía )
Theo gt : $\widehat{A}-\widehat{D}=20^{\circ}$
=> $\widehat{A}=\widehat{D}+20^{\circ}$ (*)
=> $\widehat{D}+\widehat{D}+20^{\circ}=180^{\circ}$
=> $2\widehat{D}=160^{\circ}$
=> $\widehat{D}=80^{\circ}$
Thay vào (*) => $\widehat{A}=100^{\circ}$
Ta có : $\widehat{B}=2\widehat{C}$ , $\widehat{B}+\widehat{C}=180^{\circ}$
=> $2\widehat{C}+\widehat{C}=180^{\circ}$
=> $\widehat{C}=60^{\circ}$
=> $\widehat{B}=120^{\circ}$
Vậy $\left\{\begin{matrix}\widehat{D}=100^{\circ} & & & \\ \widehat{B}=120^{\circ} & & & \\ \widehat{C}=60^{\circ} & & & \\ \widehat{D}=80^{\circ} & & & \end{matrix}\right.$